I want a smooth transition between a min and max value so that the enemy actor is able to expand and shrink (it's non-moving, just an obstacle)

Then, because multiple scaling will distort an image, you will need to keep the original image in a field; plus, you will need an int field to "time" the phases of expanding and shrinking. The value of the timer field can both regulate the speed and direction of change (expanding or shrinking). Always create a new image by copying the original image and then scale it to some new value. Basic codes would be: